Possible Scott 233A?

This is a #233 on cover with another place below for comparison. This was certed as a dull ultramarine about 50 years ago by the PF, and as a deep ultramarine by the PF around 30 years ago. I bought the cover because Jack Molesworth, one of the preeminent stamp dealers of the middle of the last century, has a long write-up on back identifying the stamp as the atypical shade of the #233a Blue. Even the experts can disagree.

"The paper of the bottom right stamp is slightly yellowed or color altered in some way."

I see this paper shade on the 4c frequently.